1. Field of the Invention
The present invention relates to a horn antenna apparatus for use in an antenna system for radar, radio communication and so forth and in the primary radiator of such an antenna system and for radiating radio waves into space with desired characteristics.
2. Description of the Related Art
FIG. 6 is a sectional view of a dual-mode type horn antenna apparatus among the conventional horn antenna apparatus shown in, for example, the xe2x80x9cSatellite Communication Technologyxe2x80x9d published by the xe2x80x9cInstitute of Electronics, Information and Communication Engineers of Japanxe2x80x9d on Nov. 10, 1980. In FIG. 6, reference numeral 1 denotes a radio-wave input portion having a circular inner waveguide diameter; 2, an aperture portion of a horn antenna for radiating radio waves into space; and 3, a tapered waveguide portion, the waveguide portion 3 is formed such that its inner diameter is increased from the side of the radio-wave input portion 1 toward the aperture portion 2. Further, reference numeral 4 denotes a higher-mode generating portion having an inner diameter that changes stepwise or in a tapered manner and used for generating higher-mode radio waves of out of the radio waves fed.
The operation of the conventional horn antenna apparatus will now be described. In the radio-wave input portion 1, radio waves having an electric field distribution in a TE11 mode as the basic mode of the circular waveguide are excited. The electric field distribution in the TE11 mode is shown in FIG. 7A. In the higher-mode generating portion 4, some of the radio waves having the electric field distribution in the TE11 mode in the radio-wave input portion 1 are converted into a higher mode such as a TM11 mode. The electric field distribution in the TM11 mode is shown in FIG. 7B. The radio waves in the dominant mode and the higher mode propagate through the waveguide portion 3 and reaches the aperture portion 2. At this time, the inner diameter xcfx86D and the taper angle xcex8 of the inner diameter of the higher-mode generating portion 4 are set at proper values so that the amplitude and phase of the higher mode such as the TM11 mode generated in the higher-mode generating portion 4 may satisfy desired values. Thus, the aperture distribution of the radio waves in the aperture portion 2 can be made to conform to an electric field distribution shown in FIG. 7C. In other words, the electric field distribution in the aperture portion shown in FIG. 7C is what is obtained by superposing the electric field distribution in the higher mode such as the TM11 mode on the electric field distribution in the TE11 mode and becomes a rotationally symmetrical electric field distribution without generating cross polarization.
As the conventional horn antenna apparatus is thus arranged, the quantity (amplitude and phase) of the TM11 Mode and the like generated in the higher-mode generating portion 4 can be set at an ideal value at a predetermined frequency of fo. However, a deviation in the quantity of the higher mode thus generated occurs when the frequency deviates from fo and while the radio wave propagating through the tapered waveguide portion 3, the phase quantity in the higher mode also deviates with respect to the dominant mode. As a result, the electric field distribution generated in the aperture portion 2 does not become rotationally symmetrical in case where the frequency is thus deviated and the problem is that the cross polarization is generated therein. In the conventional horn antenna apparatus, moreover, because a plurality of higher modes are not controllable so that an optimum quantity of, for example, the TM11 and TE12 modes is simultaneously generated in the higher-mode generating portion 4, there still exists a problem of making unattainable a horn antenna apparatus which is rotationally symmetrical and generates a smaller quantity of cross polarization over a wide frequency range.
An object of the present invention made to solve the foregoing problems is to provide a horn antenna apparatus which is rotationally symmetrical and generates a desired quantity of higher mode and a smaller quantity of cross polarization over a wide frequency range.
In order to achieve the above object, there is provided a horn antenna apparatus including: a radio-wave input portion for receiving radio waves; a waveguide portion for propagating the radio waves received; and an aperture portion for radiating the radio waves propagated by said waveguide portion into space; wherein in said waveguide portion, the inclination of a line of intersection crossing a plane including an axis in the direction of propagating radio waves with respect to said axis is not fixed but continuously varied; and wherein a distance between said line of intersection and said axis increases from the side of said radio-wave input portion toward the side of said aperture portion.
In the present invention, the distance between said line of intersection and said axis may decrease, in part of said line of intersection, from the side of said radio-wave input portion toward the side of said aperture portion.
Also, according to the present invention, there is provided a horn antenna apparatus including: a radio-wave input portion for receiving radio waves; a waveguide portion for propagating the radio waves received; and an aperture portion for radiating the radio waves propagated by said waveguide portion into space; wherein in said waveguide portion, the inclination of a line of intersection crossing a plane including an axis in the direction of propagating radio waves with respect to said axis is not fixed but continuously varied in part of said line of intersection; and wherein the distance between said line of intersection and said axis increases from the side of said radio-wave input portion toward the side of said aperture portion.
In the present invention, the distance between said line of intersection and said axis may increase, in part of said line of intersection, from the side of said radio-wave input portion toward the side of said aperture portion.
Further, according to the present invention, there is provided a horn antenna apparatus including: a radio-wave input portion for receiving radio waves; a waveguide portion for propagating the radio waves received; and an aperture portion for radiating the radio waves propagated by said waveguide portion into space; wherein in said waveguide portion, a line of intersection crossing a plane including an axis in the direction of propagating radio waves includes a plurality of straight lines; and wherein the distance between said line of intersection and said axis increases, in part of said line of intersection, from the side of said radio-wave input portion toward said of said aperture portion.